The present invention relates to a call control system in a network and, more particularly, to a call control system in an ATM switch for integrating and switching various types of information differing in traffic characteristics, such as voice, data and motion pictures, using such an asynchronous transmission mode (ATM) as in an integrated services digital network (ISDN).
With recent wide use of data communications, not only conventional voice but also important data is communicated using public switched lines. In future communications networks, therefore, high-quality transmission and switching of data are required. As a communication services network for providing not only voice and low-speed data but also high-speed data such as motion images, the broadband ISDN (B-ISDN) has entered the stage of practical application and various types of interfaces have been standardized. As opposed to conventional switching systems, the B-ISDN can equally handle information of different speeds, such as voice, low-speed data and motion images, and can also handle continuous information and burst information.
In such ATM communications the same call control method must be used for calls of different traffic characteristics. In the ATM communications, however, the transmission of channels used for communications is not periodic depending on the contents of services and the number of transmitted channels per unit time is not uniform. Thus, there is an uncertain aspect that the quality cannot be fully guaranteed even to a call connected once. In order to eliminate the uncertain aspect and control the transmission of information of different traffic characteristics with the same procedure, there is, in the ATM communications, a desire for a call control system which is distinct from that in the conventional line switching system.
In the ATM network, information of different bands is transmitted and switched in units of a fixed length which are called cells in order to be distinguishable from the conventional packet switching system. Since line data and packet data are multiplexed in cell units without distinction therebetween, the ATM network can accommodate services requiring different transmission rates with flexibility, thus permitting transmission lines to be utilized efficiently.
FIG. 1 is a conceptual diagram of communications in the ATM network. User information is divided into several cells according to its length with a header added to each cell. As an example, the header of a cell is several bytes in length and its data part is about 32 to 120 bytes in length. The header has a channel identifier, which is called a VCI, so as to identify the user that transmits the information. After being divided into cells, the user information is multiplexed on the ATM highway for transmission and switching. This switching is called self-routing because, in the ATM switching device of the network, a buffer into which the cell data is to be written is designated by a processor in the switching device using a channel number and the cell data autonomously flows through the network in accordance with the designation of the buffer. On the receiving side the required cells are extracted from the ATM highway on the basis of the channel identifier (VCI) and the header is removed to reproduce the user information.
Distinct from a conventional synchronous transmission mode (STM), the ATM system permits the same user's information to be transmitted asynchronously on the ATM highway as shown in FIG. 1. The ATM system permits voice transmit data, whose delay is not allowed, to be inserted between data in, for example, data services in which some answer waiting time is allowed.
The services performed by the ATM network includes the transmission of voice, data and motion images. The characteristic of each service and the functions required of each service will be described below.